Sadly sometimes map makers update their maps but are not changing the name. When it happens, the version on steam and on the server is not the same and this appears. If you are subscribed to this map on the workshop, unsubscribe and let it download from the FastDL. If you are not, then you may have downloaded a different version form a different server and may need to delete it from the garry's mod folder.

Note that, due to how garry's mod and FastDL works, I cannot update the map that is on the server, otherwhise every players would get that error. (The reason being that FastDL does not redownload a file that you already have, even if the version on the server is not the same)

I've already got this error with several maps on the server. Just go to the steam workshop and unsubscribe for waterworld ( in this case) like Arnold said, and It should be fine after that ( or at least it worked with my error ). ^^